The present invention relates to devices for use in the treatment of acute or chronic mono- or bi-ventricular myocardial insufficiency.
In general terms, two different kinds of device are known for the treatment of this condition.
The first kind of device is that currently known as an aortic pulsator (an I.A.B.P., that is, an intra-aortic balloon pump).
Its operation is based on the reduction of the so-called post-loading (the overall resistance of the vascular system) in synchronism with the systolic expulsion from the left ventricle in order indirectly to increase the output of the heart.
This device usually takes the form of a hollow catheter with a spindle-shaped balloon about 25-30 cm long at its end which, in its inflated condition, can occupy a volume of about 40 cm.sup.3.
The catheter in question is positioned in the thoracic aorta by being passed along the femoral artery. The catheter is connected to a driver unit with a compressor and a system for synchronising it with the patient's electrocardiogram or arterial pressure. The driver unit inflates and deflates the balloon in the thoracic aorta in synchronism with the stages of the heart's activity. In general, at the beginning of the ventricular systole (the emptying of the left ventricle and the corresponding expulsion of a quantity of blood), the intra-aortic balloon is deflated so as to reduce the telediastolic pressure (the "minimum" pressure) which is equivalent to the reduction of the peripheral resistance of the vascular system, that is, the so-called post-loading.
It is thus possible to create conditions favourable to the admission of a greater quantity of blood to the arterial system. The difference between the volume of the systolic output with and without the balloon pump represents the gain in terms of the efficiency of the system (15-20% on average). During the diastole, that is, when the aortic valve is closed, the intra-aortic balloon is inflated and consequently pushes back a quantity of blood approximately equal to its own volume (about 40 cm.sup.3) in both a cranial direction (that is, towards the upper part of the body ) and a caudal direction (that is, towards the lower part of the body). This raises the arterial pressure which has the positive result of an increase in the coronary flow-rate.
The other kind of device is essentially that currently known as a ventricular assistance device (V.A.D.). It is based on the bypassing of the right and/or left ventricles by the withdrawal of blood from the atria or directly from the ventricles and its readmission to the pulmonary artery and/or to the aorta.
Devices of this type can be divided further into alternating V.A.D.s (for example, electropneumatic, electromagnetic, electromechanical systems, etc.) which effect intake stages followed by compression stages so as to create a pulsed blood-flow, and continuous V.A.D.s which create a continuous flow by means of a system with a centrifugal pump.
The main advantage of these devices is their ability to keep a patient alive even when the heart has stopped or is fibrillating. For this reason, V.A.D. systems are used most successfully for mechanically supporting circulation whilst a heart transplant is awaited (a transplant bridge).
They have been found to be much less effective, however, when used in parallel with the heart in an attempt to compensate for temporarily weakened heart function.
A plausible explanation for this ineffectiveness is that all V.A.D.s reduce the preloading of the heart muscle but increase the aforementioned post-loading (an increase in the average arterial pressure). With an inadequate ventricle (that is, a ventricle without functional reserve) the increase in the post-loading reduces the volume of blood expelled during the ventricular systole until, in certain extreme cases of weakened contractile activity of the heart, the aortic valve fails to open (the volume of blood expelled is zero). The resulting increased post-systolic volume creates a series of physical-metabolic conditions in the ventricles which create conditions unfavourable to the functional recovery of the heart.